The testing of visual functions includes perimetry, determination of sharpness of visual perception on the retina, determination of flicker frequency, determination of contrast sensitivity, and color contrast sensitivity. Apparatus which can carry out such tests are called automatic perimeters or visual field testers and have been known in the art for many years.
In prior art visual field testers, it is common to include a hemispherical projection surface and to place a patient's eye at or close to the center of the hemisphere for testing. The tested eye is directed toward the apex of the hemisphere and, in particular, toward the apex of the internal surface of the hemisphere while the patient is directed to a central fixation illumination. A projector presents stimuli at selected points of the internal surface of the hemisphere for example, by sequentially flashing images of light sources on the internal surface of the hemisphere. In response, the patient generates signals for example, by pressing a button to indicate detection or non-detection of the stimuli. Typically, a computer receives the signals and generates the patient's visual field, which visual field is used to plot the absence, presence, and/or progress of disease. A drawback of such prior art, indirect illumination visual testers is that they are bulky and expensive. For example, in order for the patient to look directly at the internal surface of the hemisphere, the minimum distance of the internal surface from the eye must be at least 30 cm. The size of the hemisphere for a 60 degree field of view at a distance of 30 cm is 30 cm in diameter. As a rule, the diameter of the hemisphere is in a range of 60-100 cm. Further, the center of the hemisphere is normally located at eye level of the patient who is typically seated during the test procedure. Therefore, the overall height of such a prior art visual field tester is necessarily in the range of two meters.
An improvement on the above-described prior art visual field tester is disclosed in U.S. Pat. No. 5,046,835 (the '835 patent). In particular, the '835 patent discloses an apparatus used to test visual functions of a patient's eye that occupies only a fraction of the space taken up by a conventional apparatus such as that described above. In particular, the '835 patent discloses an apparatus which provides stimuli at finely spaced intervals which is comprised of a radiation source, stimuli presenting means in the form of a diaphragm (the diaphragm is disposed between the radiation source and the patient's eye), an eyepiece including one or more optical elements (the eyepiece is disposed between the diaphragm and the patient's eye), and a collection lens for producing real, intermediate images of the stimuli from the diaphragm (the collection lens is disposed in a plane between the plane of the aperture of the diaphragm and the eyepiece). The purpose of the eyepiece is to provide sharp images of the real, intermediate images upon the retina of the eye. As disclosed in the '835 patent, the apparatus further comprises a computer to set the intensity of the radiation output as a function of time. Further, as shown in FIG. 2 and as described at col. 6 of the '835 patent, an assembly 22 which includes the radiation source is movable at right angles to the optic axis of the lens of the patient's eye by a computer-controlled drive. This enables the diaphragm to move its aperture in a plane disposed at right angles to the optic axis to enable stimuli to be presented at any desired point of the plane. The coordinates of locations of successively presented stimuli are selected by the computer in accordance with a predetermined program.
A need exists in the art for a method and apparatus for testing the visual field of a patient with a compact visual field tester and, preferably, a visual field tester that projects test spots directly into the patient's eye without vignetting.